A panel is known, in which a compressive stress layer is formed on the surface of a panel to improve the strength of the surface and thereby to prevent thermal breakage during the manufacture of a cathode ray tube or to prevent breakage after the completion of a cathode ray tube. The present applicants have made various proposals relating to panels of this type (JP-A-7-21944, JP-A-7-142012, JP-A-7-142013).
In such a panel tempering process, a panel taken out from a mold, is cooled to an annealing point or lower while a large temperature difference is kept between the inside and the surface of the glass, so that a large temporary strain can be produced. However, when such cooling operation is continued in this state, the temporary strain accumulated in the panel becomes excessive, so that self-explosion may take place during the cooling process, which makes the process practically useless. Therefore, an annealing operation is conducted to relax the temporary stress to thereby ensure the practical usefulness of the process.
On the other hand, in the process of cooling the panel to an annealing point or lower to obtain a large temporary strain, a temperature distribution will be formed not only in a thickness direction but also in an in-plane direction because of the three dimensional structure of the panel such as a thickness distribution or a heat flux distribution due to air cooling. Especially at the corner portions on the inner surface of the panel face portion, the cooling rate tends to be slow as compared with the center of the face portion by a usual process due to an influence of the three dimensional structure of the panel.
In this process, the higher the cooling rate of glass, the larger the temperature gradient in the thickness direction and the higher the tempered stress. Accordingly, under such a condition, the tempered stress (the compressive stress obtainable by tempering) at the corner portions of the face naturally becomes low as compared with the center portion.
Especially when the cooling time during this process is inadequate relative to the heat capacity of the corner portions of the face, cooling of the corner portions will be inadequate relative to the center portion of the face which is sufficiently cooled to lower than the strain point. If the annealing process is carried out in a state where the temperature of the center point in the thickness of the corner portions is lower than the strain point, tempered stress of the corner portions becomes very small compared with the tempered stress of the center portion, and uniformity of tempered stress becomes worse.
Further, cooling of the corner portions on the inner surface of the face will also be slow as compared with the skirt portion. Accordingly, the tensile stress in the in-plane direction caused by the constraint of the skirt portion, will remain in addition to the above-mentioned tempered stress. This residual stress is predominantly in a mode to deform the center portion of the panel face convexly towards the inner surface side relative to the peripheral portion of the face.
Accordingly, a panel physically strengthened by a usual method tends to have a stress distribution such that the strengthening stress along the peripheral portion of the face tends to be small relative to the center portion of the face, and the strengthening stress on the inner surface of the face tends to be small relative to the outer surface of the face, due to the temperature distribution during the cooling process because of the three dimensional structure. With a panel having such a stress distribution, if the degree of the distribution is large, the effect for preventing the thermal breakage during the manufacture of a cathode ray tube is not necessarily adequate, and there has been a limit in shortening the heat treating time. Especially, in a heating process, a tensile stress is produced on the inner surface of the face, and accordingly, it is strongly desired to improve the residual stress against a low strength region on the inner surface.
Further, when this cathode ray tube is broken by an explosion proof test, broken glass fragments tend to be finely divided, and the scattering amount tends to be large, since the panel is physically tempered. Accordingly, there has been a problem that the tempered stress can not be increased indiscriminately.
Namely, in a case where crack propagation in the panel face having the above-described stress distribution during the explosion proof test, the compressive residual stress on the inner surface is smaller than that on the outer surface, whereby the energy released as a tensile stress tends to be larger on the inner surface than on the outer surface, and even when the strengthening stress on the outer surface is equal, cracking is likely to spread on the inner surface side. It is believed that consequently, an unbalance in the progress of cracking will be created between the inner and outer surfaces, whereby fine fragments are likely to be formed, and the scattering amount tends to be large.
On the other hand, the residual stress in an in-plane direction has a mode to deform the face convexly towards the inner surface side, and when this stress is released, a motion in an outward direction will be induced, which also makes the explosion proof instable. Especially with a panel which is almost flat with a small curvature of the panel face, if it is attempted to restore a deformation of the face plane towards the inner surface side created by atmospheric pressure to the initial state by reinforcement of a band, the restoration quantity of such a face plane can not be adequately taken, and the above residual stress formed in a direction opposite to the restoration direction will be a factor which makes the explosion proof instable.